Conventional articles of athletic footwear include two primary elements, namely, an upper and a sole structure. The upper is usually formed of leather, synthetic materials, or a combination thereof and comfortably secures the footwear to the foot, while providing ventilation and protection from the elements. The sole structure often incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin, relatively soft member located within the upper and adjacent the sole of the foot to enhance footwear comfort. The midsole, which is traditionally attached to the upper along the entire length of the upper, forms the middle layer of the sole structure and serves a variety of purposes that include controlling potentially harmful foot motions, such as over pronation; shielding the foot from excessive ground reaction forces: and beneficially utilizing such ground reaction forces for more efficient toe-off. In order to achieve these purposes, the midsole may have a variety of configurations as discussed in greater detail below. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear resistant material that includes texturing or other features to improve traction.
The primary element of a conventional midsole is a resilient, polymer foam material, such as polyurethane or ethylvinylacetate, that extends through the length of the footwear. The properties of the foam midsole are primarily dependent upon factors that include the dimensional configuration of the midsole, the material selected for the polymer foam, and the density of the midsole material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and energy absorption properties may be altered to meet the specific demands of the activity for which the footwear is intended to be used.